Keyed noise-clipping circuit



May 5, 1953 J. AvlNs KEYED NOISE-CLIPPING CIRCUIT Filed Deo. 15, 1950 no Z0 Z4 2 .az 5o INVENTOR IACK AVINS.

Patented May 5, 1953 KEYED NOISE-CLIPPING CIRCUIT Jack Avins, Staten Island, N. Y., assignor to Radio Corporation of America, a corporation of Dela- Ware Application December 15, 1950,' Sera1`No.200,904

(Cl. TIS-'7.5)

3` Claims. `l

The present invention relates to improvements in signal processing circuits and more particularly, although not necessarily exclusively, to signal processing circuits of the amplitude discriminatory type which permit passage of only those applied signals having amplitudes falling below a predetermined amplitude level without deleteriously affecting the waveform of the signal so passed.

The present inventionrelates more directly to a. novel signal clipping circuit particularly useful in clipping noise disturbances from electrical signals having recurrent portions of the same relative or nominal amplitude.

In one of its aspects the present invention wheny applied to the television receiving art provides a novel noise limiting circuit which permits all -noise in excess of synchronizing signal peaks to be efciently clipped without destroying the waveform .of rthe. synchronizing Signal. In particular the novel precision noise clipping action provided bythe presentinvention is rendered substantially immune to rather large variations n the peak to peak amplitude of the applied signal.

The present invention therefore further relates to the improvements in the design and manufacture oftelevision receiving circuits so as to allowa greater dynamic range of television `signal amplitudes Ito be successfully received and reproduced and so as to provide a greaternoise immunity in this signal reproduction.

Perhaps nowhere in the electronic art is the demand greater for `precision signal processing at low costs than in the manufacture of television receivers. As is well-known by those'skilled in the art theV present daystandard television signal comprises two Vmain portions which are distinguishable from one anotherby both waveform and amplitude.

4'These portionsl are usually called the synchronizing component andthe video component. It is the purpose of thesynchronizing component toV maintain synchronism between the television transmitter image scanning device and the television receiver Signal reproducing devicey sof'that the picture elementsk dened by the transmitter will be properly positioned in the. .television re` ceiver picture raster. The video component` deW iined by lower amplitude variations of course represents the brightness of each element in the picture asscanned at the transmitter `and as re production at the receiver takes place.

In television receiving circuits some form of synchronizing signal separator circuit (usually called sync separator circuit) is employed toseparate the synchronizing signal vcomponent (termed sinc) from the composite television signal. This synccomponent is then applied to suitable time constant networks which allow the line and" frame frequency sync pulsesA to be separately channeled to the horizontal and vertical deflection generators of the television receiver. Any noise appearing on .the top of the sync pulses will, of course, tend to mis-synchronizeI the deflection circuits and therefore cause undesirable disturbances in the reproduced image.

In theV prior art considerable noise immunity in the deflection circuits of the televisionreceiver has been obtained lthrough the careful choice of circuit values used in the design of the sync separator circuit itself. For example, `the sync separator circuit generally takes the form of a threshold device'which permits passage of only signal information above a certain predetermined amplitude. VInthe case of the standard 'television signal the synchronizing component is transmitted atmaximum on percent television carrier amplitude while .peak video information ranging from white to Vblack inthe image isv normally transmitted within the limits of 15 percent and 75 percentof the maximum television ,carrier amplitude. Hence 'for optimum sync separator action the threshold for the separator must be continually varied in accordance with received signal vstrength so as to permit only the upper 25 percent of the received signal (representing the sync) topass to the deiiection circuit. Noise peaks therefore tend. .to make the signal appear stronger than it actually is and therefore cause improper sync separation.

The prior art` attemptsy at solving this problem have been numerous. One of themost. common remedial devices is to pass the received demodulated television signal through a clipping stage which clips all signalsin excess of a value defined by a control voltage. This control voltage is made a function of the amplitude reached by recurrent peaks of sync signal (representing 100 percent carrier). The developmentof this control voltage is most commonly'accomplished by rectifying the demodulated video signal so that a capacitive load in the rectifier circuit will develop` an average voltage representing the peaks of sync. The problem here, however, .is vthatrn order to. develop the required` control voltage in this manner a considerable amount of energy must bek taken from the received video signal. This means that in cases where the video, signal is developed across high impedance output terminais the energy taken from the signal at peaks of sync to develop this control voltage will tend to distort or squash the waveform of the sync signal. In many instances the destruction of the waveform of the sync is as deleterious to the quality of the reproduced picture as if no attempt at noise immunization had been made.

Moreover, it is apparent that in carrying out the prior art noise clipping process the control voltage referred to above must in itself be relatively noise immune otherwise a rather unsatisfactory situation would arise in that the level at which the signal is clipped to eliminate noise is in turn dependent upon the noise itself.

Thus it is an object of the present invention to provide an improved signal processing circuit of the noise clipping variety for use in signal processing systems of the radio receiving type.

It is another object of the present invention to provide a new and improved noise clipping circuit for use in television receiving circuits which permits precision noise 4clipping at the peaks of the received sync without adversely iniiuencing the waveform of the sync.

It is another purpose of the present invention to provide an improved noise clipping circuit for use in television signal channels adapted to separate the sync component from received composite television signal and especially in which the composite television signal is developed across a high impedance output terminal across which conventional noise limiting systems would produce distortion of the sync waveform.

In the realization of the above objects and features of advantage with particular attention being given to television receiving systems, the present invention contemplates the use of a keyed signal amplifier having a storage capacitor connected in its output circuit. Received television signal is then applied to the input of the amplier stage and the amplier stage is in turn keyed on only during the arrival of incoming sync signals. The voltage developed across the storage capacitor then represents the amplitude of incoming sync signals and is relatively immune from noise. The voltage across the storage capacitor is then used as a reference voltage against which a diode may work to clip noise peaks in excess of the peaks o sync derived from the same signal source from which the amplier is fed. Since the energy for charging the capacitor is not derived from the television signal source considerably less sync waveform distortion is observed than in some prior art systems.

A better understanding of the operation of the present invention, as well as a fuller appreciation of its objects and features of advantage will be obtained through a reading of the following description especially when taken in connection with the accompanying single iigure of the drawing in which there is shown a combination block diagram and schematic representation of a television receiving system in which the present invention finds embodiment.

Turning now to the drawing there is shown at IB a television R. F. tuner followed by an in termediate frequency amplifier I2. The television R. F. tuner receives signals from the antenna I4 which are amplied and superheterodyned between the tuner andintermediate frequency amplier and iinally demodulated by the diode '16.' The demodulated video signal i8 then appears at the terminal 2J of the diode load. The terminal is connected with the grid 22 of 4 the first video amplier 24 whose output at terminal 25 is D. C. coupled to the grid 23 of the second video amplier 30. The cathode 32 of the first video amplifier 24 is connected through resistance 3ft to the contrast control potentiometer 38 whose Variable tap 13S is grounded. Proper operating voltages for the D. C. coupled ampliers 2d and Sil are, of course, provided by suitable connections tc the bleeder i2 connected from power supply terminal 44 to ground. The capacitive inductance circuits 45 and E3, as well as inductances E@ and 52 provide well-known frequency compensation in the video ampliers.

In accordance with the present invention the output terminal 5d of the second video amplier 3i] is connected to the control electrode 5S of an electrcn discharge tube 58. Tube 58 is connected as a cathode follower type amplier and by way of example only is shown as a triode. The cathode e0 of tube 58 is connected with ground through the resistor 52, video amplifier load resistor til, inductance 52 and hence through the bleeder d2. If desired, the resistor $2 may be connected directly to ground. The anode G6 of tube 5t is, in accordance with the present invention, connected to a source of positive going pulses of the type shown at 58. The pulses G8 should be synchronously related with the sync pulses of the arriving television signal and hence may be conveniently derived from the iiy back pulses produced in the horizontal deflection crcuit l0. This technique for deriving keying pulses of this type is well known in the art and is shown by way of example in an article entitled Radio Set and Service Review in the Radio Electronics magazine, November 1950, pages-34l through 36. In the iigure this is accomplished by means of a tap resistance or potentiometer 12 connected in shunt with the primary 16 of the deflection output transformer 75, The exact method of obtaining the keying pulses 63 forms no part of the present invention. The pulses 68 are made of suiiicient amplitude to cause the tube 58 to be driven into conduction during the interval of the keying pulses.

In further accordance with the present invention a storage capacitor 18 is connected from cathode 60 of discharge tube 58 to ground. A diode 8i) is then connected between the output terminal 54 and the cathode 6U of the discharge tube 53 through dropping resistor 82. A noise clipped video signal is then conducted from a connection to resistor 32 by a circuit path 84 to the input of any conventional sync separator circuit such as, for example, discharge tube 86 with its associated time constant networks 88 and 90. This general type of sync separator circuit as well as other suitable sync separator circuits are shown and described in a publication entitled Basic Television Principles and Servicing by Bernard Grob, published by the McGraw Hill Co., New York, New York, June 1949. Horizontal sync information is applied through diierentiating network 92 to the input of the horizontal deflection circuit 'l0 while vertical synchronizing information is applied through the integrating network 96 to the input of the vertical deflection circuit E8. The outputs of both the horizontal and vertical deflection circuits are of course both approximately connected to the deiiection yoke llJ associated with the kinescope m2. The cathode (not shown) 0I the kinescope E92 may be coupled with the terminal 54 of the video amplifier tube 33 and video signal modulation of the electron beam in the kinescope.

`racer;

Witli=thexception cffithemovel noise'fclippi-ng circuitioitlietpresentfinventionfshowndn tl'i'cwdotL tedflinaiareaaldi offithxergu i-rL cuitrelementithus:farrdesoriber `as-"wel-llasA su ab] e circuits; forifthesblock elements' 'shownff are indi)- catedrini articleientitle d iTelevisionfReceii/ersi bygAnthonvRriwrightxappeartng in tlielRCiifeiview;;for.f:Ma-rch l 9411;: and'alsozin: theiaboverf-refe erencedxRndi .i Electronics a'gazine:article?v Thernovelfpperatipn.rofl ..erpresent invention, however' isf substantially asifoll hws. Inzthe :gure the composite..televisionfzsignal l .lie .aocearing at thesoutputfterminal:51 offgth'e-fseccnd-:-videoafname plier'g 3f! contains several 4types oF-"informa'ticn Among .-these'f the.irst-fisrsynehronizing signal ,irrformationlrepresented'by thasync peaksll Inf; .the seccndis video signal; information@etermined vby the...w.avef.orm vIHisappearing?,between the sync peaks.;l Ihird1y.blackdeiel:information is represented ibyg thexheight of the fiblankinef pulses'A .I i 6v. Fourthly, the signalcl will vha'vean A; C. axis such as- I lil vwhose.;distance.- frein the level of blanking. i l tirepresents,the,-averagezbrightness fof the; scene` being` transmitted. (see Television Standards Waveform.1950pages 1253, November 1950 issue of theIREProceedngs). This cmeans that, the vaverage .,D..L C., potential.- of. the output terminal. 51!! yv'illlyarywith.respecttofground v.or a Xedyol-tagedatum in accordance with the average brightness of the scene transmitted. Furthermore, inrtheparlticular.arrangementshown in the ligure, since there is a ,direct current coupling from the outputroii the diode l'to'the output terminal- 54, the averageimg. potential" l#the terminal ilcvvithA respect-1 .tcngronnd willialso vary iniaccordance with received signaal-strengths This latter function is, of course, not necessary for the successful practice of the present invention. As will be seen, it is suicient that D. C. potential of the output terminal 54 vary only in accordance with average scene brightness.

Thus in accordance With the present invention the waveform |08 is applied to the grid 56 of the keyed amplier 58 through resistance H I. Since the amplifier 58 is keyed on only during the positive going extremities of keying pulses 68 (which are in turn in synchronism with the sync pulses Ill! of the signal |08) the voltage across the capacitor 'i8 will always be representative of peaks of sync. This or course is in accordance With well known cathode follower action, the larger the resistance 62 is made the closer the terminal voltage of capacitor E8 will equal the true peak voltage reached by the sync l lil, Since the diode Sil is connected from the terminal 54 to the upper terminal of capacitor 'i8 it will only conduct when the anode of the diode 8D tends to go. more positive than the voltage across the ca pacitcr i8. This of course occurs only when noise disturbances tend to create peaks in the terminal 54 which are in excess of peaks or sync lill. Thus diode 8U will conduct only on noise peaks and its conduction current will produce a heavy voltage drop across the dropping resistor 82 so as to produce a very effective noise clipping action. Since the output terminal 51S is connected to the input of the ampliner 58 no energy is required from the second video amplifier to charge the capacitor i8. This charging energy comes solely from the deilection circuit through the agency of keying pulses 68. Since the storage capacitor 'i8 is in the cathode of the amplier 58 the capacitor may be made quite large due to the low charging impedance it sees. Hence noise pulses that are icy-passed or clipped by the diode 80 will not-tendttoivary Athereferencef2potential or control potential developed Aacrossf'capait'orr'l Duewto'the'fabove mentlonedffarct-thatithefpresent "inventiorr `requires fno-energy--fper seif rom fthe video ampliiier'tof'develp theerei-eren'ce-potential againstwhichi noiseclippingiactioiris referenced, the co mmonlyL undesirable: prior artAA problem-f" of sync squashing"" is largely Y* elirninatedf Sync s'quashingf" 'cti-course vins-prior yart Aarrangements occurred', as described??herei-naboveff duef to' the rather:'night-impedance fnature-I of the-widen kam'. pli'erwoutput rcircuit'rfso that 'de'mandsf -of 'l' energy in: order. to. charge 1' acapacitorf.` directlysthrough rectification@causedfserious:voltage -dropsrini the videof amplifier outputfcircuitv andffhence squash-A ingi 'or distortionofithe isync Isignali- Sincef thef'amplier recfis -rconduoting V`crrlyior the i short duration. `oif ith lkeying lpulse; Sii'correspending tto; theiarrival of'syncI information; -it'can beiseen :,thatithe? reference.'y or-reontroly voltage deL veloped across;ztheicapacitcr.fiiiiis-irelattvelyinoise immunesi. exit isilnotfresponsivertolthefocourrence of noise'fsignal;b'etwe'ensyncpulses...

Itiiis'- thusilfseen frcmathe i above Athat f the noise limiting tcircuit'ofnth'e pre'sentiinventi'on williprof' videiautcmaticfcompensationsfor charges irrree ceivedsignalxlevelf'soithatnoiseiclippingrisfalways imposeduatfthefpeak ,cfs sync,twitltoutiproducing distortion:ofuthesynchrcnizing It` will fbe; understood 2in the-'rpractice f of the above finventiorrrits'isinet fnecessarycto a make Hout.; puti-terminalzl reflectfbothcarrierstrength and DWG.' p icturefincrmaticnn Henceitfis noti'necese sary to provide complete D. C. coupling from the demodulator diode I6 to the input of the noise immunizing circuit shown in the dotted line area |64. It is only desirable that some means be provided for clamping or otherwise establishing D. C. picture information at terminal 5ft.

It is obvious that the utility of the signal processing circuit of the present invention is in no Way limited to television signal channels. Most any electrical signal having a recurrent maximum amplitude portion which nominally represents datum signal information will be of a nature to benefit from the circuit of the present invention.

Having thus described my invention what I claim is:

1. In a television receiver adapted to receive television signals having a recurrent synchronizing pulse component and comprising a video amplifier having an output terminal, a keyed noise clipper comprising: a unilateral conduction device having an anode, a cathode, and a control electrode, a point of reference potential, a capacitor connected between the cathode of said unilateral conduction device and said point of reference potential, a direct current path including a resistance connected between the cathode of said unilateral conduction device and said point of reference potential, means connecting the output terminal of said video ampliiier to the control electrode of said unilateral conduction device so as to apply to said control electrode television signals whose recurrent synchronizing pulse component extends in a positive direction, rectifier means including an anode and a, cathode connected between the output terminal of said video amplier and the cathode of said unilateral conduction device with the cathode and the rectifier means connected to the cathode of said unilateral conduction device, and means to apply to the anode of said unilateral conduction device keying pulses synchronous with the recurrent pulse component of the television signals, said keying pulses signal waveform.

having a positive polarity withrespect to said point of reference potential.

- 2. In a television receiver adapted to receive television signals having a recurrent synchronizing pulse component and comprising a video amplifier having an output terminal, a keyed noise clipper comprising: a unilateral conduction device having an anode, a cathode, and a control electrode, a point of reference potential, a capacitor connected between the cathode of said unilateral conduction device and said point of reference potential, a resistor connected inv shunt with said capacitor, means including a resistor for connecting the output terminal of said video amplifier to the control electrode of said unilateral conduction device so as to apply to said control electrode television signals whose recurrent synchronizing pulse component extends in a positive direction, rectier means including an anode and a cathode connected between the output terminal of said video amplier and the cathode of said unilateral conduction device with the cathode of said rectier means connected to the cathode of said unilateral conduction device, and means to apply to the anode of said unilateral conduction device keying pulses synchronous with the recurrent pulse component of the television signals, said keying pulses having a positive polarity with respect to said point of reference potential.

3. In a television receiver adapted to receive television signals having a recurrent synchronizing pulse component and comprising a video amplifier having an output terminal, a keyed -noise clipper comprising: a normally non-conductive unilateral conduction device having an anode, a cathode, and a control electrode, a point or" reference potential, a capacitor connected between the cathode of said unilateral conduction device and said point of reference potential, a direct current path including a resistance connected between the cathode of said unilateral conduction device and said point of reference potential, means connecting the output terminal of said video amplifier to thev control electrode of said'unilateral conduction device so as to apply to said control electrode television signals whose recurrent synchronizing pulse component extends in a positive direction, a rectifier having an anode and a cathode, a resistor connected between the output terminal of said video amplifier and the anode of said rectier, a connection between the cathode of said rectier and the cathode of said unilateral conduction device, and means to apply to the anode of said unilateral conduction device keying pulses synchronous with the recurrent pulse component of the television signals to render conductive said normally nonconductive unidirectional conduction device during the occurrence of said keying pulses, said keying pulses having a positive polarity with respect to said point of reference potential.

JACK AVINS.

References Cited in the le 0f this patent UNITED STATES PATENTS Scoles A Aug. 21, 1945 

